The present invention relates in general to automotive assemblies comprising sheet molding composites, and, more specifically, to improved adhesive bonding of a sheet molding composite (SMC) element to other elements.
Sheet molding composite (SMC) materials have been used and are being investigated for making various kinds of structural automotive components such as body panels (e.g., hoods, fenders, decklids, pickup boxes, pillars, lift gates, and roofs), engine components (e.g., valve covers and oil pans), vehicle frame elements, bumper beams, fan shrouds, and many other types of components. The use of SMC materials has many advantages over steel and other types of composite material such as hand-laid fiberglass including lower weight, greater consolidation of parts, ability to use less complex and expensive tooling for molding the parts, greater range of component styling, and short cycle times for the molding processes.
An SMC sheet or body is comprised of a sandwich formed with a resin paste and chopped glass fibers between sheets of carrier films. Typically, one film is advanced under a box of resin paste which is deposited onto the film. Glass fibers are threaded through a chopping machine which cuts pieces of predetermined lengths and randomly deposits them onto the advancing film and resin paste. A second film having additional resin paste deposited on it is laid atop the first film and compacted into a sandwich that may be rolled up or folded for later use in a compression mold to form an SMC molded part.
A typical resin paste may be comprised of a filler (such as calcium carbonate), polyester, a lower profile additive (such as polyvinyl acetate or urethane), catalyst, a pigment, and other agents. Due to the initial placement of the fibers at the center of the sandwich, they generally remain at the interior portions of a molded part away from the surface.
A typical SMC element molded as part of an automotive assembly includes a functional section (e.g., for providing a Class A visible surface of a panel) and an attachment section for joining the SMC element to other parts within an assembly. One desirable manner for joining an SMC element to other elements is by adhesive bonding. In order to obtain good adhesion, however, a bonding pad on the SMC element for receiving adhesive must usually be pre-treated because the smooth surface on a typical SMC bonding pad after compression molding lacks good adhesive characteristics.
Compression molding tools for SMC components typically employ highly polished tool surfaces to achieve sufficient Class A surface quality. Chrome is usually deposited on the surfaces to achieve a high wear capability during the molding process. Chrome helps resist the highly abrasive nature of the SMC material during molding.
The smooth surface of an SMC bonding pad has been prepared for bonding by various pre-treatment methods known in the prior art, including electrical discharge treatment, plasma treatment, flame treatment, laser etching, gritblasting, sanding, and washing with solvents. Such treatments improve joint strength obtained by adhesive bonding. However, they result in undesirable costs, additional processing, and environmental consequences. It would be desirable to increase joint strength without requiring any such pre-treatments.